1. Field of the Invention
The present invention relates to an automatic switching system forming a communication network as a switching node, and more specifically to the automatic switching system having a redundant system configuration in order to improve a reliability of the communication network.
2. Description of the Related Art
As an automatic switching system having a redundant system configuration in order to improve a reliability of the communication network, which is composed of a plurality of automatic switching systems as switching nodes and inter connected by transmission lines to each other, there are known ones such as disclosed in Japanese Patent Laid-open Nos. Sho 63-276952 and Hei 5-244225, for example as the conventional technology.
FIG. 6 is a block diagram schematically showing a switching system disclosed in Japanese Patent Laid-open No. Sho 63-276952. As illustrated, a switching system 800 generally comprises a switching network 801, a current-system control section (ACT) 802 and an auxiliary-system control section (STAND-BY) 803.
The ACT system control section 802 comprises an ACT system central controller 811 connected to the switching network 801 through a signal line 831, an ACT copy controller 812 connected to the ACT system central controller 811 through a signal line 832 and an ACT memory 813 connected to the ACT copy controller 812 through a signal line 833. On the other hand, the STAND-BY system control section 803 comprises a STAND-BY central controller 821 connected to the switching network 801 through a signal line 841, a STAND-BY copy controller 822 connected to the STAND-BY central controller 821 through a signal line 842 and a STAND-BY memory 823 connected to the STAND-BY copy controller 822 through a signal line 843.
As described above, in this switching system 800, the switching network 801 is separated from the control sections 802, 803, and the control sections 802, 803 are forming a redundant configuration which provides a back up operation for each other in case of one section having a faulty condition. Then, a bus 851 is extended between the ACT central controller 811 and the STAND-BY central controller 821, and a bus 852 also is extended between the ACT copy controller 812 and the STAND-BY copy controller 822, whereby data to be written in the STAND-BY memory 823 is passing from the ACT central controller 811 through the bus 851, the STAND-BY central controller 821, the signal line 842, the STAND-BY copy controller 822 and the signal line 843 when a bus trouble occurs between the ACT copy control apparatus 812 and the STAND-BY copy controller 822, for example. Thus, there can be improved reliability against the bus trouble.
FIG. 7 is a block diagram schematically showing a communication system disclosed in Japanese Patent Laid-open No. Hei 5-244225, and illustrates connection relationships of its internal processing blocks. As illustrated, a communication system 900 generally comprises a current-system communication controller (ACT) 901, an auxiliary-system communication controller (STAND-BY) 902 and a selector 903, in which the communication controllers 901, 902 for processing data communication control in accordance with a predetermined communication protocol are redundantly provided as the ACT system and the STAND-By system.
The ACT communication controller 901 comprises an ACT read/write control unit 911 and an ACT protocol processing section 912 connected to the ACT read/write control unit 911 through a signal line 951. Further, the ACT protocol processing section 912 comprises a processor 921 connected to a bus 950 through a signal line 952 and a memory 923 connected to the bus 950 through a signal line 953. Similarly, the STAND-BY communication controller 902 comprises a STAND-BY read/write control unit 931 and a STAND-BY protocol processing unit 932 connected to the STAND-BY read/write control UNIT 931 through a signal line 961. Further, the STAND-BY protocol processing section 932 comprises a processor 941 connected to a bus 960 through a signal line 962 and a memory 942 connected to the bus 960 through a signal line 963. Then, the ACT read/write control unit 911 and the STAND-BY read/write control unit 931 are connected to each other by means of a state information transfer bus 970.
The ACT read/write control unit 911 reads out state information of layer 2, which is necessary to the minimum to continue a layer 2 (data link layer) processing, from a memory 922 provided in the ACT protocol processing section 912 through the signal line 951, the bus 950 and the signal line 953, and transfers this state information of layer 2 from a state information transfer bus 970 to the STAND-BY read/write control unit 931. The STAND-BY read/write control unit 931 writes the state information of layer 2 transferred through the state information transfer bus 970 in a memory 942 provided in the STAND-BY protocol processing unit 932 through the signal line 961, the bus 960 and the signal line 963.
The ACT communication protocol processing section 912 and the STAND-BY communication protocol processing section 932 are adapted to carry out a data communication control in accordance with a procedure of an HDLC (High level Data Link Control Procedure) as an example of a communication protocol. The selector 903 selects any one set of a reception line 971 and a transmission line 972 of the ACT communication controller 901 and a reception line 973 and a transmission line 974 of the STAND-BY communication controller 902, and connects the selected set to an external reception line 975 and an external transmission line 976.
FIG. 8 is a diagram used to explain the procedure in which the communication system 900 is operated, and illustrates a processing procedure executed when the redundant system (ACT and STAND-BY) is switched over in response to a command entered by an operator or the like. Assuming that there occurs a system switching factor at a timing shown by an open arrow X in the figure, then the ACT communication controller 901, which has so far received a frame signal F from a communicated party, transmits a communication restriction frame signal RNR instructing a pause of a frame transmission to the communicated party, thereby controlling a data transmission of the communicated party (see 1+L in FIG. 8).
Subsequently, the communication system executes a reception processing of the frame signal F which has been so far transmitted until the communicated party receives the frame signal RNR (see 2+L in FIG. 8). Then, the ACT read/write control unit 911 transmits state information SS read out from the memory 922 to the STAND-BY read/write control unit 931 (see 3+L in FIG. 8). After that, the STAND-BY read/write control unit 931 writes the received state information SS in the memory 942. Thus, the protocol processing states in the ACT protocol processing section 912 and the STAND-BY protocol processing section 932 become equivalent so that the STAND-BY protocol processing section 932 becomes able to continue the layer 2 processing.
Then, by exchanging a control signal CC between the ACT system and the STAND-BY system, the system is switched over (see 4+L in FIG. 8), and a communication is resumed by using the STAND-BY communication controller 902 as a new ACT communication controller. Specifically, the STAND-BY communication controller 902, which became the new ACT communication controller, transmits a communication restriction releasing frame signal RR for instructing a resumption of a frame transmission to the communicated party (see 5+L in FIG. 8), and receives the frame signal F continuously transmitted from the communicated party, thereby resuming a communication (see 6+L in FIG. 8).
However, in the switching system shown in FIG. 6, since the switching network 801 is a single configuration, and not made redundant, there is then the problem that a trouble brought about by the switching network 801 itself will affect service to the communication network users. Furthermore, in this switching system 800, even though the switching network 801 is made redundant, the redundant switching network 801 and the control sections 802, 803 for controlling the redundant network are separated from each other with the result that the ACT system and the STAND-BY system are switched separately. Then, the interface and the protocol exchange between the ACT system and the STAND-BY system become complicated. There is then presented a new problem that a scale of the system becomes large in size.
Moreover, in the communication system shown in FIG. 7, when the system is switched over from the ACT communication controller 901 to the STAND-BY communication controller 902, the protocol processing states in the ACT protocol processing section 912 and the STAND-BY protocol processing section 932 are made equivalent by transferring the state information SS stored in the memory 922 within the ACT protocol processing section 912 to the memory 942 within the STAND-BY protocol processing section 942. There are then presented the following problems.
Specifically, when a sudden trouble occurs relative to the ACT system, there is then the possibility that even data containing connection information with a communicated party or the like will be lost. If so, then state information must be frequently exchanged between the ACT communication controller 901 and the STAND-BY communication controller 902. There is then the risk that a service interruption such as a line disconnection will occur inevitably.